Object- and location-based inhibition of return to superimposed surfaces

Previous studies have suggested the existence of spatial and object-based Inhibition of Return (IOR) effects, and proposed that they are driven by separate mechanisms. These studies have exclusively used objects occurring in spatially separate locations. Thus the object-based effects could be mediated by a location-based mechanism. To control for location, we superimposed two objects (random dot kinetograms). This study examines whether IOR is present for objects that are superimposed or requires that the objects are separated in space. We modified the traditional dynamic IOR displays (Tipper et al, 1991) by placing 2 superimposed surfaces in each of two peripheral locations (left vs right). Location-based IOR was observed regardless whether the target appeared on the cued or uncued surface. Critically, object-based IOR was not present; instead we found evidence of object-based facilitatory effects. Thus location-based but not object-based IOR is found with superimposed surfaces. In Experiment 2, we asked whether spatial separation is necessary throughout the trial or at time of cueing. These results have implications for the relative roles of subcortical oculomotor (e.g. superior colliculus) and cortical substrates for mediating IOR.